/Engine/Source/Runtime/Engine/Classes/GameFramework/PlayerController.h
This is meant ot provide a mechanism for client side prediction to correlate client input and server frame numbers. Frame is a loose concept here. It doesn't necessary mean GFrameNumber. Its just an arbitrary increasing sequence of numbers that is used to label disrete units of client->Server input. For example the main thread may tick at a high variable rate but input is generated at a fixed step interval.
struct FInputCmdBuffer